1. Field of the Invention
This invention relates to the field of hollow, metallic waveguide transmission line components operating in the millimeter wave region of the frequency spectrum and more particularly to an improved tetrahedral junction type of waveguide switch which is especially suited for use in millimeter wave applications, such as radar equipment and the like, for example.
2. Description of the Prior Art
Tetrahedral junction waveguide switches are used in many millimeter wave applications employing the hollow, metallic waveguide transmission medium to modulate or control the flow of electromagnetic wave energy through the waveguides. This type of switch basically comprises two lengths of hollow, metallic waveguide which are axially aligned with each other along their longitudinal axes. Each waveguide length has a reduced width end which is rotated 90 degrees with respect to the reduced width end of the other waveguide length and the reduced width ends are juxtaposed so that the lengths are mutually cross-polarized with respect to signal transmission through the aperture formed by the juxtaposed ends of the waveguide lengths. A rod of a ferrite material having gyromagnetic properties is disposed in the aperture and aligned with the longitudinal axes of the waveguide lengths. The rod has tapered ends which cooperate with the orthogonally disposed reduced width ends of the waveguide lengths to load or terminate the waveguide lengths and thereby to cut-off signal transmission through the aperture. However, when the ferrite rod is subjected to a unidirectional magnetic field along its longitudinal axis, the permeability of the rod is changed and the signal applied to the switch is rotated 90 degrees in accordance with the well-known Reggia-Spencer effect so that the signal is permitted to pass through the aperture. Accordingly, by controlling the magnetization of the ferrite rod, the switch may be placed in either the cut-off or transmission states.
When tetrahedral junction waveguide switches are used as crystal protectors for millimeter wave radar receivers, they must protect the receiver during the duration of the radar transmitter pulse and must therefor be in the cut-off or "reflective" state. When the radar echo signal is received, they must be in the transmission state. The time required to operate the switch from the cut-off state to the transmission state should be kept as short as possible so as not to significantly reduce the minimum range of the radar. In order to minimize the drain on the dc power supply of the radar system, a permanent magnet structure is employed to magnetically bias the ferrite rod into the switch transmission state. An electromagnetic coil arrangement is then employed to induce a counter balancing magnetic field in the ferrite rod when it is desired to trigger the switch into the cut-off state during the radar echo receiving state.
The permanent magnet structures which have been employed for the aforementioned purpose have generally been large, bulky offset ring magnets which have significantly increased the size and weight of the waveguide switch and also the radar equipment in which the switch is used. Additionally, it has been noted that the offset ring magnets were subject to a degradation of performance because of the successive operation of the electromagnetic coil used to activate the switch. The magnetic field produced by these magnets also did not subject the ferrite rod to a uniform magnetic field in the axial direction of the ferrite rod which was necessary for optimum performance.